The effect of granulocyte-colony stimulating factor (G-CSF) or macrophage-colony stimulating factor (M-CSF) on patients with malignant lymphoma was analyzed. G-CSF was administered for ten days after conventional chemotherapy causing an increase in the granulocyte and monocyte counts. The increase in these cells shortened the duration of a leukocyte count lower than 2000 or 3000/mm3. There were no detectable effects from G-CSF on other blood cells. M-CSF had no effect on any of these cells. Receptors of these cytokines on various types of leukemic cells were also analyzed by flow-cytometry using fluorescent isothianate-labelled G-CSF or M-CSF. With this simplified method, G-CSF receptors were detected on almost all of the myeloid acute leukemia cells, but not on the nonmyeloid leukemic cells. M-CSF receptors were also detected on all of the monocytic acute leukemic cells, but not on myelocytic or lymphocytic leukemic cells.